In cellular communications systems implemented in accordance with American National Standards Institute (ANSI) J-STD-008, all base station digital transmissions are referenced or synchronized to a common system-wide clock that is synchronous with Global Positioning System (GPS) time. As used here, synchronized or synchronous events means that two or more events happen at the same frequency with a constant relative phase. One of the synchronized signals transmitted by each CDMA base station is a pilot channel.
A pilot channel is an unmodulated, direct-sequence spread spectrum signal transmitted so that a personal station or subscriber unit may acquire the timing of the forward CDMA channel, may obtain a phase reference for coherent demodulation, and may measure signal strength of a reference signal for comparing relative signal strengths between base stations.
The pilot channel assists the subscriber unit in initial acquisition of CDMA signals of the cellular system when the subscriber unit is powered up, and helps the mobile compare signal strength of signals from different base stations in order to determine when to handoff. Pilot signal strength may be defined as the ratio of received pilot energy to overall received energy, or Ec/Io.
The pilot channel is designed to be easy for the subscriber unit to find. Thus, it is the simplest code channel from a base station; all base stations use Walsh code 0 (all ones), and no long code. The pilot PN sequence is a pair of modified maximal length PN (pseudnoise) sequences with period 2.sup.15, which is used to spread the forward CDMA channel and the reverse CDMA channel. The pilot channel sequence repeats 75 times every 2 seconds, or every 26.667 milliseconds. The pilot channel is also the strongest (most energy) code channel sent from the base station.
To enable the subscriber unit to distinguish pilot channels from different base stations, the pilot PN sequence is offset in time by an offset index, which is measured in units of 64 PN chips of a pilot PN sequence, relative to the 0 offset pilot PN sequence. The offset index may also be referred to herein as a frame offset, which is an offset in time that is less than the duration of a frame.
When a subscriber unit is powered up it uses a searcher to find a pilot channel. Once a pilot channel is found, the subscriber uses the pilot PN sequence rollover point as a reference point to demodulate the sync channel using Walsh code 32. Once a complete sync message is recovered and decoded, the PN offset information is used to synchronize the subscriber unit with system time. Next, the subscriber unit receives and demodulates information on a paging channel, which contains information to help the subscriber unit find pilot channels from nearby base stations, with which the subscriber unit may handoff. This information is typically organized in a neighbor list, which informs the subscriber unit of pilot PN sequence offset indexes of the nearby base stations. Thus, once the subscriber unit has completed the processing of the initial signal acquisition, the subscriber unit uses pilot channels identified in the neighbor list to determine the best base station for maintaining a communication link.
As mentioned previously, pilot channels from different base stations are identified by an offset index, which may also be called a frame offset. This method of identification works because all base stations are synchronized to system time. Benefits to having base stations synchronized to system time include the ability to implement a relatively simple searcher in the subscriber unit, the ability to quickly set the searcher to the PN sequence offset index of a neighboring pilot channel for measuring signal strength, and the ability to measure time of flight and time of arrival for use in subscriber location finding systems.
However, synchronizing base stations is not accomplished without cost. Base stations are typically synchronized with GPS receivers, which add to the cost of the base station. Additionally, some base stations may be located so that GPS satellite signals are not available for reception at a point close to the base station. For example, if a CDMA base station is located in a subway, GPS signals cannot be easily received. Thus, there may be a need for an unsynchronized base station in some CDMA cellular communications systems.
If an unsynchronized base station is used in a CDMA system with synchronized base stations, and the unsynchronized base station uses the common PN spreading sequence used by all other synchronized base stations, the subscriber unit may no longer be able to properly identify pilot channels from base stations by determining the PN sequence offset index of the pilot signal. This is because the PN sequence of the unsynchronized pilot channel will drift relative to system time, causing it to periodically appear to be one or another of the synchronized base stations in the system.
Thus, to distinguish pilot channels of unsynchronized base stations from pilot channels of synchronized base stations, and pilot channels of unsynchronized base stations from one another, the unsynchronized pilot channel uses a pilot PN sequence that is different from a common pilot PN sequence used by all synchronized base stations. Furthermore, as compared to one another, the unsynchronized pilot PN sequences must be different from those used in adjacent cells in order to distinguish closely spaced unsynchronized pilot channels from each other. Typically, several unsynchronized pilot PN sequences are chosen (perhaps as many as 32 different sequences) so that a subscriber unit may be able to perform measurements and handoff between adjacent unsynchronized cells. The number of unsynchronized pilot PN sequences should be large enough to support an acceptable PN sequence reuse pattern should a service area of contiguous unsynchronized cells be needed.
A draw back of mixing synchronized and unsynchronized cells in a CDMA cellular system is that the pilot channel searcher in the subscriber unit has to be more complicated because it no longer searches for a common pilot PN sequence. A pilot searcher that searches for many pilot PN sequences at many frame offset indexes would be a slow searcher, which would slow access to the system and handoffs between cells.
With reference now to FIG. 1, there is depicted a pilot PN sequence for use in a cellular communications system wherein all base stations are unsynchronized. This pilot PN sequence is described in a paper entitled "Fast Cell Search Algorithm in DS-CDMA Mobile Radio Using Long Spreading Codes" by K. Higuchi, M. Sawahashi, and F. Adachi, published in 1997 in the IEEE Vehicular Technology Conference Proceedings, pp. 1430-1434. As illustrated, pilot PN sequence 20 uses a plurality of M-chip markers 22 spaced evenly at N number of locations in pilot PN sequence 20. The system disclosed in the article is not compatible for integration into a synchronized cellular communications system.
Therefore, a need exists for an improved method and system for integrating synchronized and unsynchronized spread spectrum signals in a cellular communications system.